Abstract: Disclosed is a system for replacing dirty aquarium water with fresh, clean aquarium water and making up for evaporation, thus keeping the aquarium water level constant. The inventive system includes a passageway for supplying clean water to an aquarium, which passageway has a bypass port leading to a throat designed to cause a pressure reduction. The throat is fluidly connected to an aquarium outlet located at the surface of the aquarium so that the reduction in pressure at the throat causes a suction effect to remove dirty water from the surface of the aquarium. Downstream of the throat, the dirty water is conveyed to a waste outlet.